Liquid cooled electrical machine

ABSTRACT

Flow control apparatus for an electrical machine and comprising an arrangement of shaped chambers and passages for conveying a liquid coolant. The rate of heat transfer from certain portions of the machine to the coolant is determined by the varying velocity of the liquid through the chambers, resulting in a generally uniform cooling of those portions of the machine.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/063,451 entitled LIQUID COOLED ELECTRICAL MACHINE filed Mar. 25,2011, which claims priority to International Application No.PCT/GB2009/051114 filed on Sep. 3, 2009, which claims priority to GreatBritain Patent Application No. 0816711.6 filed on Sep. 12, 2008.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention relates to the cooling of components in anelectrical machine. In particular the invention relates to apparatus forcontrolling the flow of a coolant so as to provide generally evencooling of a portion of an electrical machine.

As the demand for electrical power in vehicles has increased, the trendin alternator (generator) design has been toward greater power capacity.At the same time there has been a requirement to make the alternatorhousing more compact, such that it takes up less space in the vehicle'sengine bay. The result of these requirements has been an increaseddemand on the alternator's cooling system, which must remove the heatgenerated by the stator/rotor windings which are located in thealternator housing.

An alternator cooling system typically comprises a housing including oneor more passages through which a liquid coolant flows. The coolant isusually diverted to the alternator from the vehicle's engine. U.S. Pat.No. 6,633,098 describes an arrangement for cooling the stator and rotor(including windings on the rotor) in a high-output alternator. Thestator is supported by brackets which constitute a sleeve structure.Coolant passages run axially along the sleeve and circumferentially atits ends. Coolant enters at one end of the sleeve and follows a circuitwhich takes it to the other end and back again. It then exits thesleeve, having cooled the stator. The rotor is cooled by a centrifugalfan. Fins and guides are provided for directing the flow of air from thefan.

U.S. Pat. No. 6,046,520 is also concerned with cooling a high-outputalternator. A pot-like shell (essentially a tube with a base) isattached to the alternator housing, forming a gap between the alternatorhousing and the shell. This gap extends circumferentially around, andaxially along, the housing. Coolant enters the gap at an inlet which istangential to the shell. It passes along the gap around thecircumference of the shell and exits from an outlet about 300 degreesaround from the inlet. The gap between the inlet and outlet (60 degreesapart) is partially blocked to resist the fluid taking the shorter routeto the outlet. The inlet and outlet are axially displaced (staggeredalong the length of the shell) to enhance cooling. There is arestriction about 180 degrees around the circumference of the shell.This restriction causes some of the liquid to be diverted axially to thebase area of the shell, where it can cool the bearing in that area. Therest of the liquid passes through the restriction and onto the outlet.

Thus, general arrangements of coolant passages for alternators areknown. However, a growing need for the reduction of fuel consumption bythe internal combustion engine has led to the development of theIntegrated Starter Generator (ISG), which represents an alternative tothe conventional alternator. Like an alternator, the ISG generateselectric power when the engine is running, for supplying the vehicle'selectrical system and charging its battery. However, the ISG combinesthe functions of the conventional alternator/generator with those of thestarter motor in a single ISG. Thus, it is capable of switching from analternator mode to a starter mode. The ISG can automatically stop andthen rapidly restart the engine to avoid periods of unnecessary engineidling, such as when a vehicle is waiting at a traffic light. Thisreduces fuel consumption and exhaust emissions.

Like an alternator, the ISG includes a stator and rotor which need to becooled. However, the dual function of the ISG described above means thatit requires other components in addition to those usually found in analternator. In particular, the ISG includes various electricalcomponents for producing the high current needed for starting theengine. Furthermore, complex electronics are necessary to controlefficiently the start-stop function of the ISG. Moreover, the ISG facesthe same requirement for compactness as the conventional alternator,hence the electrical and electronic components should be integrated intothe housing of the ISG in the smallest possible volume. This combinationof high-power components in a small, enclosed space means that the ISGgenerates a large quantity of heat—significantly greater than that of aconventional alternator—which must be efficiently removed in order forthe ISG to operate effectively.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is providedflow control apparatus for an electrical machine, the flow controlapparatus comprising a first wall and a second wall, the second wallbeing inclined with respect to the first wall so as to define a taperedfirst chamber between the first and second walls, the first chamberincluding an entrance aperture and an exit aperture, wherein in use thetaper causes the velocity of a liquid in the first chamber to be variedas the liquid is conveyed between the entrance aperture and the exitaperture, such that a transfer of thermal energy from a heat source tothe liquid via the first wall is substantially uniform between theentrance aperture and the exit aperture.

Preferably the second wall of the flow control apparatus comprises aguide member, the guide member having a first face and a second face,the first face being inclined with respect to the second face so as toform a tapered profile between the first and second faces, and whereinthe tapered first chamber is defined by the first wall and the firstface of the guide member. The guide member may be a separable componentof the machine, potentially simplifying the manufacture of the structureof the machine itself. Furthermore a range of guide members may beproduced, each with a different taper angle. This enables the selectionof an appropriate guide member which will provide the required flowvelocity profile and cooling performance for any particular machine.

The flow control apparatus may further comprise: a third wall comprisinga flow path, the flow path being arranged to receive a liquid; and asecond chamber, defined by the third wall and the second face of theguide member, the exit aperture of the first chamber being coupled tothe second chamber, wherein in use the liquid is conveyed from the firstchamber to the second chamber via the exit aperture and at least aportion of the liquid in the second chamber is received diverted ontothe flow path. Single or multiple flow paths serve to distribute theliquid to other parts of the machine for cooling those parts.

The first wall of the flow control apparatus may comprise an undulatedsurface suitable for inducing turbulence in the liquid. Alternatively oradditionally, the first face of the guide member may comprise anundulated surface suitable for inducing turbulence in the liquid.Turbulent flow is advantageous because it can produce a higher rate ofheat transfer compared to laminar flow.

According to a second aspect of the present invention there is provideda generally cylindrical housing for an electrical machine, the housingcomprising: a first end comprising the flow control apparatus describedabove; a second end; an inner annular wall and an outer annular wall,the annular walls extending axially between the first and second ends; aplurality of passages between the inner and outer annular walls, thepassages extending axially between the first and second ends; and aconduit, extending circumferentially between the inner and outer annularwalls and joining the axial passages at the second end, wherein in useliquid is conveyed by the flow path of the flow control apparatus intothe axial passages at the first end to the conduit at the second end,such as to transfer thermal energy from a heat source to the liquid viathe inner annular wall. The features of the housing offer the combinedadvantages of control of flow velocity profile and distribution ofliquid for cooling.

According to a third aspect of the present invention there is provided agenerally ring-shaped guide member for use in a cavity of an electricalmachine and suitable for influencing the flow of a liquid between anentry aperture and an exit aperture in the cavity of the machine, theguide member comprising a first face and a second face, the first facebeing inclined with respect to the second face so as to form a taperedprofile between the first and second faces, wherein in use, a taperedfirst chamber is defined between a first wall of the electrical machineand the first face of the guide member, and wherein the taper of thefirst chamber causes the velocity of liquid adjacent to the first faceto be varied as the liquid is conveyed between the entrance aperture andthe exit aperture. The guide member may be a separable component of themachine, potentially simplifying the manufacture of the structure of themachine itself. Furthermore a range of guide members may be produced,each with a different taper profile. This enables the selection of anappropriate guide member which will provide the required flow velocityprofile and cooling performance for any particular machine.

The first face of the guide member may comprise an undulated surfacesuitable for inducing turbulence in the liquid. Turbulent flow isadvantageous because it can produce a higher rate of heat transfercompared to laminar flow.

According to a fourth aspect of the present invention there is provideda mating member for an electrical machine, the mating member comprisinga rigid inner core surrounded by a resilient outer covering, thecovering comprising: a first mating portion having a generally plainsealing surface; a second mating portion having a generally textureddamping surface; and an inner annular wall and an outer annular wall,the annular walls extending between the first and second matingportions, the mating member being arranged to engage between twoportions of the housing of an electrical machine such that in use thesealing surface inhibits leakage of liquid from the housing and thedamping surface absorbs vibration transmitted between the portions ofthe housing. The dual-function mating member provides an efficient meansboth for sealing the liquid coolant passages and reducing vibration inthe machine.

Preferably the textured damping surface comprises a plurality of raiseddimples. The dimples are deformable such that they allow the twoportions of the housing to be clamped securely together without causingthe resilient covering to be crushed or otherwise significantlydistorted in a radial direction (inwards or outwards).

According to a fifth aspect of the present invention there is provided agenerally cylindrical housing for an electrical machine, the housingcomprising a first portion, the first portion comprising: a first endand a second end; an inner annular wall and an outer annular wall, theannular walls extending axially between the first and second ends; aplurality of passages between the inner and outer annular walls, thepassages extending axially between the first and second ends; and aconduit, extending circumferentially between the inner and outer annularwalls and joining the axial passages at the second end, the housingfurther comprising: a second portion; and a mating member as describedabove, the mating member being arranged to engage between the first andsecond portions of the housing. The features of the housing offer thecombined advantages of control of flow velocity profile and distributionof liquid for cooling; sealing; and reduction of vibration.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be moreparticularly described, by way of example, with reference to theaccompanying drawings in which:

FIG. 1 shows an integrated starter generator machine (ISG) for theengine of a vehicle;

FIG. 2 shows a front elevation of the ISG of FIG. 1;

FIG. 3 shows the interior of the housing of the ISG of FIG. 1;

FIG. 4 shows a splitter ring for use in the ISG of FIG. 1;

FIG. 5 a shows upper and lower views of the splitter ring of FIG. 4.FIGS. 5 b and 5 c show sectional views of the splitter ring;

FIG. 6 shows a cutaway view of half of the housing of FIG. 3, includingthe splitter ring of FIG. 4;

FIG. 7 shows a sectional view of the ISG of FIG. 1;

FIG. 8 shows a flow path of a liquid coolant over the lower face of thesplitter ring of FIG. 4;

FIG. 9 shows a flow path of a liquid coolant over the upper face of thesplitter ring of FIG. 4;

FIG. 10 shows an NVH (noise, vibration and harshness) ring for use inthe ISG of FIG. 1;

FIG. 11 shows the NVH ring of FIG. 10 installed in the ISG of FIG. 1;

FIG. 12 shows a modified housing used in another embodiment of thepresent invention; and

FIG. 13 shows a modified splitter ring for use with the housing of FIG.12.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an integrated starter generator machine (ISG) 1 for theengine of a vehicle. The ISG 1 is shown in front elevation in FIG. 2.The ISG 1 is a switched reluctance motor comprising a brushless motorwith a dedicated electronic controller. It is driven by a belt (notshown) connected between the pulley 3 and the crankshaft of the engine.Torque for starting the engine is produced by the magnetic attraction ofa steel rotor to stator electromagnets. No permanent magnets are neededand the rotor carries no “squirrel cage” or windings. The 12 V ISG 1will deliver up to 65 Nm cranking torque, 2.4 kW cranking power and 3 kWgenerated power.

The ISG 1 comprises a generally cylindrical, die-cast aluminum housing 5with a front face 7 and a rear face 9. The housing 5 accommodatesprimarily the stator assembly and associated driveshaft and bearings. Analuminum front cover 11 is removably attached to the front face 7 of thehousing 5. A pod 13 containing electrical and electronic components isremovably attached to the rear face of the ISG 1. The rear portion ofthe housing 5 projects axially inwards to form an external cavity (notshown) which receives the pod 13.

The ISG 1 includes cooling apparatus with inlet and outlet tubes 15, 17.The cooling apparatus utilises liquid coolant from the engine forcooling the electrical and electronic components and the statorassembly.

FIG. 3 shows the housing 5 with the front cover 11 and the pod 13omitted. The interior of the housing 5 is cast with a total of 15 ribs19 extending axially from their roots near the rear of the housing 5towards the front, terminating short of the front face 7. The ribs 19are of unequal length and are arranged such that, viewed 180 degreesaround the circumference of the housing 5 from the outlet tube 17, theirforward most portions extend increasingly closer towards the front face7. The ribs 19 are circumferentially spaced such as to form between thema plurality of axial channels 21. Toward the rear of the housing 5 is acast ridge 23 extending circumferentially around the interior of thehousing 5 and joining the roots of the ribs 19 together. The ridge 23projects inwardly to form a narrow step 25 at the root of each rib, thestep 25 being normal to the face of the rib 19. The ridge 23 furtherforms a recessed ledge 27 in each axial channel 21 between adjacent ribs19. The recessed ledges 27 are rearward of the steps 25, hence thealternately spaced recessed ledges 27 and steps 25 form a castellatedpattern.

Toward the rear of the housing 5 and adjacent to the ridge 23 is apartition wall 29. The partition wall 29 is normal to the longitudinalaxis of the housing 5 and is rearward of the recessed ledges 27 of theridge 23. The pod 13 (not shown) containing the electrical andelectronic components is on the other side (i.e. rearward) of thepartition wall 29. The pod 13 is separated from the partition wall 29 bya thermal mat.

FIG. 4 shows a splitter ring 101, constructed from an injection-moldedplastics and for use in the ISG 1. FIG. 5 a shows upper and lower viewsof the splitter ring 101. FIG. 5 b shows a sectional view taken alongline B-B of FIG. 5 a. FIG. 5 c shows a sectional view taken along lineC-C of FIG. 5 a.

The splitter ring 101 comprises a generally flat upper face 103 and apartially contoured lower face 105. An inlet 107 and an outlet 109,suitable for conveying a liquid, are positioned at opposite sides of thesplitter ring 101 (i.e. 180 degrees around from each other).

The lower most portion of the lower face 105 forms a flat, horizontalbase 111 for the splitter ring 101. Contoured portions 113, 115 extendtoward the inlet 107 and the outlet 109 respectively from the horizontalbase 111.

The upper face 103 is inclined with respect to the horizontal base 111,such that the upper face 103 slants downwards from the thicker sectionof the splitter ring 101 at the inlet 107 to the thinner section at theoutlet 109. Thus, as shown in FIG. 5 b, the profile of the splitter ring101 is generally tapered between the inlet 107 and the outlet 109.

Studs 117, positioned intermittently between the inlet 107 and theoutlet 109, extend from the upper face 103. The studs 117 increase inlength from the inlet 107 to the outlet 109, such that the top surfacesof the studs form a horizontal plane (parallel with the horizontal base111). Studs 117 also extend from the lower face 105 in the region of theoutlet 109.

Circumferentially spaced lugs 119 extend radially outwards from thesplitter ring 101. The lugs extend vertically from the upper face 103towards the lower face 105 such that the lower surfaces of the lugs 119form a horizontal plane.

FIG. 6 shows a cutaway view of half of the housing 5 of FIG. 3,including the splitter ring 101. The splitter ring 101 is installedforwards of the partition wall 29 and adjacent to the ridge 23. Thelower face 105 of the splitter ring 101 faces the partition wall 29. Thestuds 117 extending from the lower face 105 contact the partition wall29 and serve to prevent the thin end of the splitter ring 101 fromflexing. The lugs 119 of the splitter ring 101 fit into and aresupported by the recessed ledges 27, such as to terminate the axialchannels 21. The inlet 107 of the splitter ring 101 is aligned with theinlet tube 15 of the housing 5.

FIG. 7 shows a sectional view of the ISG 1 taken along line A-A of FIG.2. The stator assembly 301 is installed into the housing 5 subsequent tothe fitment of the splitter ring 101. The stator assembly 301 iscontained within a cylindrical steel sleeve 303 which has an outer wallwith an upper end 305 and a lower end 307. The outside diameter of thesleeve 303 is greater than the inside diameter of the housing 5. Duringthe process of assembly of the ISG 1, the housing 5 is heated such thatit expands to accommodate the sleeve 303. The sleeve 303 is theninserted into the housing 5 so that its lower end 307 rests on and issupported by the steps 25 at the roots of the ribs 19. The lower end 307of the sleeve 303 also contacts the ends of the studs 117 of the upperface 103 of the splitter ring 101, constraining the splitter ring 101 toprevent axial movement. The outer wall of the sleeve 303 contacts thefaces of the ribs 19 around the circumference of the interior of thehousing 5. The upper end 305 of the sleeve 303 extends towards the frontface 7 of the housing 5, beyond the forward most portions of the ribs19, thereby joining the axial channels 21 together to form acircumferential conduit 311 between the housing 5 and the outer wall ofthe sleeve 303, forward of the ribs 19.

Thus, the axial channels 21 between the ribs 19 are closed off along thefaces of the ribs 19 by the outer wall of the sleeve 303, but remainopen both at the front face 7 of the housing 5 and at the roots of theaxial channels 21 where they intersect with the splitter ring 101.

Subsequently, during the installation process the housing 5 is allowedto cool and consequently contracts, thereby forming an interference fitbetween the faces of the ribs 19 and the outer wall of the sleeve 303.This interference fit ensures that the housing 5 and sleeve 303 are notseparated from each other when the ISG 1 experiences high temperaturesin normal service.

With the splitter ring 101 and the stator assembly sleeve 303 installedin the housing 5, there are defined two chambers, each bounded at theinner edge of the splitter ring 101 by the inwardly projecting portionat the rear of the housing 5. A lower chamber is defined between thepartition wall 29 and the lower face 105 of the splitter ring 101. Dueto the contoured portions 113, 115 extending from the horizontal base111 of the lower face 105, the cross-sectional area of the lower chambervaries along the splitter ring 101 from the inlet 107 to the outlet 109.

An upper chamber is defined between the horizontal lower end 307 of thestator assembly sleeve 303 and the slanted upper face 103 of thesplitter ring 101. Due to the slant of the upper face 103, thecross-sectional area of the upper chamber changes at a constant ratebetween the inlet 107 and the outlet 109 of the splitter ring 101.

The front cover 11 is installed on the housing 5, closing off thecircumferential conduit 311 at the front face 7 to produce a taperedsection in the circumferential conduit 311, the tapered section beingdefined by the forward most portions of the ribs 19 which extend towardsthe front face 7 as described above.

With the ISG 1 so assembled: the inlet tube 15 of the housing 5 islinked to the lower chamber at the inlet 107 of the splitter ring 101;the lower chamber is linked to the upper chamber at the outlet 109 ofthe splitter ring 101; the upper chamber is linked to the axial channels21 at the regions of the lugs 119 which are received in the recessedledges 27; the circumferential conduit 311 is linked to the axialchannels 21 at the upper end 305 of the sleeve 303; and the outlet tube17 of the housing 5 is linked to the circumferential conduit 311.

Thus, there is formed a passageway for a liquid to flow through thehousing 5 of the ISG 1 for cooling the electrical and electroniccomponents and the stator assembly.

In use, the engine's water pump forces liquid coolant under pressurethrough the inlet tube 15 and into the lower chamber via the inlet 107of the splitter ring 101. At the inlet 107 the coolant divides into twoflow paths, each path tracing a semi-circle around the partition wall29. One of these coolant flow paths 123 is illustrated in FIG. 8, whichshows one half of the lower face 105 of the splitter ring 101 of FIG. 4.The two flow paths rejoin each other in the region of the outlet 109 ofthe splitter ring 101.

In the coolant flowing along each path 123 there exists a velocityboundary layer extending from the surface of the partition wall 29 intothe flowing coolant. Another velocity boundary layer extends into thecoolant from the lower face 105 of the splitter ring 101.

The coolant is an incompressible liquid which behaves according to theprinciple of continuity, so under steady flow conditions the velocity offlow is inversely proportional to the cross-sectional area at the liquidface (i.e. the area of a plane taken across the lower chamber normal tothe direction of flow of the coolant). Thus, the velocity of the coolantincreases as it flows from the inlet 107 and along that narrowing,tapered section of the lower chamber which is defined by the contouredportion 113 of the lower face 105 of the splitter ring 101 (which isabove the partition wall 29). The coolant then flows at a constantvelocity through that constant cross-section of the lower chamber whichis defined by the horizontal base 111 of the lower face 105. Thevelocity of the coolant then decreases as it flows through thatwidening, tapered section of the lower chamber which is defined by thecontoured portion 115 of the lower face 105 of the splitter ring 101.The coolant then reaches the outlet 109 of the splitter ring 101. Thevelocity of the coolant through the lower chamber is approximately 0.2to 0.55 meters per second at a volumetric flow rate of approximately 2liters per minute. This is optimal in terms of heat extraction from theelectrical and electronic components. It also reduces the possibility ofcoolant contaminants becoming embedded in this critical cooling area.

The hot electrical and electronic components in the pod 13 adjacent tothe partition wall 29 transfer heat to the partition wall 29 whichresults in a temperature difference between the partition wall 29 andthe coolant flowing over it. Consequently there exists a thermalboundary layer extending from the surface of the partition wall 29 intothe flowing coolant, and there is a transfer of thermal energy from thepartition wall 29 to the coolant. The rate of heat transfer isproportional to the difference in temperature between the partition wall29 and the coolant. The dominant heat transfer mechanism is convection.More particularly, it is a process of forced convection because thecoolant is being pumped through the lower chamber by the engine's waterpump.

As described above, the variation in the cross-sectional area throughthe lower chamber causes the velocity of the coolant to vary as it flowsfrom the inlet 107 to the outlet 109 of the splitter ring 101.Accordingly, the amount of thermal energy extracted by the coolant perunit time varies between the inlet 107 and outlet 109. That is, the rateof heat transfer from the partition wall 29 to the coolant is defined bythe shape of the lower chamber and it is kept substantially constantbetween the inlet 107 and the outlet 109 of the splitter ring 101.Consequently, the partition wall 29 is maintained at a substantiallyuniform temperature, avoiding any significant temperature gradient inthe electrical and electronic components. This is important because sucha temperature gradient could lead to uneven and possibly inadequatecooling of the electrical and electronic components, thereby limitingthe performance of the ISG 1 or even causing damage to it.

The heat emitted by the electrical and electronic components in the pod13 at the rear of the housing 5 reaches about 400 W. The heat emitted bythe stator assembly in the housing 5 reaches about 1200 W.

The amount of heat convected away from the partition wall 29 to thecoolant can be found, for example, by the application of Newton's law ofcooling. This requires a knowledge of the convective heat transfercoefficient, which is a function of several factors including liquiddensity, velocity, viscosity, specific heat and thermal conductivity.The convective heat transfer coefficient may be obtained by firstcalculating the Nusselt number (the ratio of the heat entering thecoolant from the partition wall 29 to the heat conducted away by thecoolant) and the Prandtl number (the ratio of the velocity boundarylayer thickness to the thermal boundary layer thickness). Alternatively,convective heat transfer coefficients for common fluids and various flowgeometries have been established by experimentation and are well known.

At low velocities the flow of the coolant tends to be laminar. At highervelocities, with correspondingly higher Reynolds numbers, the flow canbe turbulent. Turbulence influences the thermal boundary layer as wellas the velocity boundary layer, because turbulence affects enthalpytransfer just as it does momentum transfer. The stagnant liquid filmlayer at the hot surface of the partition wall 29 is thinner in aturbulent flow compared to a laminar flow. Consequently, the convectiveheat transfer coefficient, and therefore the rate of heat transfer, isgreater in the case of turbulent flow. Hence it is desirable tointroduce turbulence into the coolant to increase the rate of heattransfer from the partition wall 29 to the coolant. This improvescooling performance such that the electrical and electronic componentsmay be maintained at the desired temperature while operating at higherpower levels. Alternatively, depending on the operator's requirements,they may be operated at lower temperatures at the same power level,potentially prolonging their useful service life.

The introduction of turbulence is achieved by the inclusion ofirregularities in the surface of the partition wall 29, as can be seenin FIG. 3. These irregularities take the form of a series of alternatingpeaks and troughs, which present a wave-like pattern when the partitionwall 29 is viewed in profile. These undulating peaks and troughs perturbthe flow so as to alter the slope of the temperature profile of thecoolant near the surface of the partition wall 29. They also have theeffect of intermittently enlarging and reducing the cross-sectional areaof the liquid face, thereby causing local variation in flow velocity.However, it is the effect on velocity of the contoured portions 113, 115extending from the horizontal base 111 of the lower face 105 that isdominant.

The inclusion of the irregularities in the surface of the partition wall29 generally increases the thickness of the section of the partitionwall 29, providing increased structural strength in this region of thedie-cast housing 5. This is beneficial because it helps the wall toendure the conditions to which it is subjected. These conditions includehigh temperatures and a large number of hot/cold thermal cycles, inconjunction with vibration caused by the rotation of the rotor andmovement of the vehicle to which the ISG 1 is attached. However, despitethe advantage of greater strength, the thickness of the wall iscarefully limited to ensure that its heat transfer characteristics areconsistent with the cooling requirements of the electrical andelectronic components in the pod 13.

In general, a consequence of turbulence in the flow of liquid in adevice is that the overall pressure drop through the device tends toincrease. In the case of the present invention such a pressure drop isundesirable because it would place a greater demand on the engine'swater pump which might, in turn, increase the engine's fuel consumption.Therefore, although it is advantageous to introduce turbulence into theflow as described above, the amount of turbulence is consciouslylimited. Thoughtful design produces effective mixing of the coolant toaid heat transfer without producing an unacceptably large pressure dropthrough excessive turbulence, which might occur, for example, if thecoolant were forced to change direction too suddenly. In particular,careful determination of the gradient of the undulating peaks andtroughs at the surface of the partition wall 29 enables the flow controlapparatus to cool the electrical and electronic equipment effectivelywhile strictly limiting any potential fuel consumption penalty.

Having reached the outlet 109 of the splitter ring 101 the coolant thenflows into the upper chamber and onto the upper face 103, where onceagain it divides into two flow paths. One of these flow paths 125 isillustrated in FIG. 9, which shows one half of the upper face 103 of thesplitter ring 101 of FIG. 4.

As described above, the cross-section of the upper chamber varies at aconstant rate due to the slant of the upper surface 103 of the splitterring 101 with respect to the lower end 307 of the stator assembly sleeve303. As the coolant flows along the upper face 103 away from the outlet109, the cross-section of the upper chamber becomes narrower, hence thevelocity of the coolant increases.

As the coolant flows along the upper face 103 a portion of the coolantis diverted onto another flow path 129 into the axial channels 21. Thisportion of the coolant is conveyed by the axial channels 21 to thecircumferential conduit 311 and extracts thermal energy from the hotstator assembly sleeve 303. The coolant then flows through the wideningsection of the circumferential conduit 311 towards the outlet tube 17,from which it leaves the housing 5 to return to the cooling system ofthe engine of the vehicle.

Thus, the coolant is distributed at the desired velocity and quantityaround the periphery of the housing 5 via the various axial channels 21,thereby cooling the stator assembly.

FIG. 10 shows an NVH (noise, vibration and harshness) ring 501 for anISG 1 as described above. FIG. 11 is a detailed view of the portion ofthe ISG 1 indicated in FIG. 7 and shows the NVH ring 501 installed inthe ISG 1 between the housing 5 and the front cover 11. The NVH ring 501separates the front cover 11 from the housing 5 in such a way that thetwo are not in direct contact with each other.

The NVH ring 501 has two functions. Firstly, it acts as a seal toprevent the escape of liquid coolant from the passages in the housing 5.Secondly, it absorbs and damps out vibration induced in the housing 5 bythe stator/rotor assembly, thereby insulating the front cover 11 fromthe rotating components and reducing noise.

The NVH ring 501 comprises a stiff, steel core 503 enclosed in anelastomeric (rubber) casing 505. The steel core 503 is robust andprovides form to the relatively flexible and deformable casing 505. Thecasing 505 has upper and lower faces 507, 509. The lower face 509 isflat and smooth. The upper face 507 includes a plurality of raiseddimples 511 spaced apart from each other. The upper and lower faces 507,509 are connected by inner and outer annular walls 513, 515. The walls513, 515 include circumferential gripping grooves 517 which correspondto radial projections 519 on the housing 5 and front cover 11. Thegrooves 517 engage the projections 519 such that the NVH ring 501resists axial movement relative to the housing 5 and provides a leaktight seal.

A lip 521 extends radially outward from the upper face 507 to define aflange 523 around the periphery of the outer annular wall 515. Theflange 523 abuts annular projections 525 on the housing 5, locating theNVH ring 501 and providing a contact surface for the front cover 11.

The front cover 11 is secured to the housing 5 by a plurality of bolts.The axial clamping load transmitted to the NVH ring 501 as the bolts aretightened causes the dimples 511 to undergo generally elasticdeformation in the inward and outward radial directions. This flatteningof the dimples 511 absorbs the clamping force such that the front cover11 and the housing 5 are connected securely together without causing theannular walls 513, 515 of the NVH ring 501 to be crushed or otherwisesignificantly distorted. This serves to ensure the integrity of theseal, which prevents an escape of liquid from the cooling passages ofthe ISG 1.

It will be appreciated that the flow control apparatus described abovemay take various forms and these are not limited to those expressed inthe drawings.

The irregularities in the surface of the partition wall 29 could bedimples, grooves or other appropriate means for perturbing the flow suchthat it becomes turbulent.

The splitter ring 101 may be constructed from a material other thanmolded plastic, for example aluminum or steel.

In a second embodiment of the present invention the splitter ring 101 isomitted. Instead, the variations in cross-sectional area of the coolantflow path described above are obtained by forming the structure of theupper and lower chambers in the casting of the housing 5 itself.

FIG. 12 shows a housing 701 used in a third embodiment of the presentinvention. The housing 701 is similar to the housing 5 described abovewith the exception that the number of ribs 703 is increased from 15 to29, with a corresponding increase in the number of axial channels 705between the ribs 703.

FIG. 13 shows a splitter ring 901 for use with the housing 701 of FIG.12. The splitter ring 901 is similar to the splitter ring 101 describedabove with the exception that the number and position of the lugs 903and studs 905 is altered to correspond to the housing 701.

In this embodiment the greater number of ribs 703 in the housing 701means that the width of each rib 703 and each axial channel 705 issmaller. This offers three particular advantages. Firstly, there is adecrease in the level of stress in the housing 701 where the housing 701is in contact with the stator assembly sleeve 303. Secondly, resonancein the axial channels 705 (caused by the stator/rotor) is reduced,lessening the noise level of the ISG 1 and extending the fatigue life ofthe housing 701. Thirdly, due to an increase of rib 703 surface area incontact with the coolant in the axial channels 705, there is improvedheat transfer from the stator assembly sleeve 303 to the coolant. Thisresults in a more uniform temperature field around the circumference ofthe housing 701 and a lower peak temperature.

The invention claimed is:
 1. A generally ring-shaped guide member whenused in an electrical machine a cavity of an electrical machine andsuitable for influencing the flow of a liquid between an entry apertureand an exit aperture in the cavity of the machine, the guide membercomprising: a first face and a second face, the first face beinginclined with respect to the second face so as to form a tapered profilebetween the first and second faces wherein; in use, a tapered firstchamber is defined between a first wall of the electrical machine andthe first face of the guide member, and wherein the taper of the firstchamber causes the velocity of liquid adjacent to the first face to bevaried as the liquid is conveyed between the entrance aperture and theexit aperture.
 2. The guide member as claimed in claim 1 wherein thefirst face comprises an undulated surface suitable for inducingturbulence in the liquid.